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Image of the Week

A new Panoramic Sky map of the Milky Way's Stellar streams

Stellar stream structures in the northern Galactic sky, as obtained by the STREAMFINDER algorithm after processing the Gaia data. Every point on this map is a star, where the associated color represents their flow speeds. A rich network of criss-crossing streams can be seen. In addition of detecting previously known streams, for example the so called "GD-1" stream, the researchers also reported several streams as new discoveries, suffixed here with "*". Image credit: Khyati Malhan, Rodrigo A. Ibata, Nicolas F. Martin

This initial mapping of the stellar substructures in the galactic halo starts to unravel the complex formation history of our Galaxy. The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. One of the prevailing frameworks that attempts to explain the structure and the composition of the Milky Way galaxy, known as the hierarchical structure formation framework, suggests that the Milky Way halo was built up over an extended period through the aggregation and accretion of smaller mass systems (such as satellite galaxies).

Numerical simulations based on this cosmological paradigm have shown that when low mass stellar systems orbiting the host galaxy come too close to the host’s centre, they undergo disruption and distortion due to the action of tidal forces. This process rips out stars from the progenitor and leads to the formation of stellar residues that are called "tidal debris’". A series of such merging and accretion events in the environment of the galaxy, collectively, gives rise to an inhomogeneous distribution of stars in the galactic halo. If this theoretically-motivated hierarchical cosmological model is indeed correct, then it is expected to see some fossil residues of these past or on-going accretion events.

With the arrival of Gaia DR2, Khyati Malhan along with his PhD supervisor Dr. Rodrigo A. Ibata (CNRS, DR2) and researcher Dr. Nicolas F. Martin (CNRS, CR), built a state-of-the-art algorithm to analyse the Gaia data. STREAMFINDER is a new powerful algorithm that was designed to detect stellar streams in the Milky Way halo. Stellar streams are a class of stellar debris that are formed when the tidal disruption acts slowly. These structures are of extreme astrophysical importance, as they serve as sensitive seismographs allowing us to probe the underlying dark matter distribution of the Milky Way galaxy, and are also crucial as tracers of the galaxy formation process. The algorithm was constructed to hunt for groups of stars in the dataset that resembled such streams, sharing similar orbits and also containing particular types of stellar populations.

Potential streams of stars identified by STREAMFINDER in the Milky Way's inner halo, from 5 to 15 kpc. Results for a metal-rich selection are shown in the top panels, while results for intermediate metallicity are shown in the lower panels. Some of the previously known stream structures can be seen in these maps, for example the so called "GD-1" stream, Indus and Jhelum. All stream points displayed here have detection significance of more than 5 σ. Image credit: Khyati Malhan, Rodrigo A. Ibata, Nicolas F. Martin

On the very day of the data release, 25th April 2018, these researchers launched STREAMFINDER on the Gaia data. This study reveals, for the first time, an all-sky structural and kinematic map of the stellar streams of the Milky Way halo. They found a rich network of criss-crossing streams, often with striking kinematic coherence. Several of these structures, which can be seen in the above figures named Gaia -1, 2, 3, 4 and 5, were reported as discoveries. The lumpiness in these maps strongly supports the picture in which the Milky Way accumulated stars by undergoing a significant number of merging and accretion events. While the results that they obtained are but a first step in the comprehensive mapping of the Milky Way’s stellar halo, they already show the promise of the Gaia mission, borne out by the rich view of the heavens it has unveiled.

Stellar stream structures in the southern Galactic sky, as obtained by the STREAMFINDER algorithm after processing the Gaia data. Every point on this map is a star, where the associated color represents their flow speeds. A rich network of criss-crossing streams can be seen. In addition of detecting previously known streams, for example the "Sagittarius" stream, the researchers also reported several streams as new discoveries, suffixed here with "*". Gaia-2, for example, is a newly discovered stream that can be seen to be lying right at the southern Galactic cap. Image credit: Khyati Malhan, Rodrigo A. Ibata, Nicolas F. Martin

Update on 18 April 2019

A new article "The Streams of the Gaping Abyss: A population of entangled stellar streams surrounding the Inner Galaxy" is now accepted for ApJ which is available here. Below an image is given which gives an overview of the streams found up till now: